This invention relates generally to pipeline construction and more particularly concerns equipment used in joining sections of pipe end-to-end.
Known conventional designs of fusion machine heaters include mica/foil heaters, cast tubular heaters and machined plates with bent tubular heaters, all of which are very expensive. Mica/foil heaters are very difficult to design and are useful only on relatively small heaters. Cast tubular heaters do not permit zoned heat distribution and cannot be repaired. Bent tubular heaters do not permit zoned heat distribution and tend to break at the bends due to thermal expansion differentials.
A fusion heater was also proposed 35 years ago which would use a plate with a central opening and metal-cased heaters snugly held in radial bores extending on the central plane of the plate from its outer to its inner circumferences. The bores would be equally radially displaced in an effort to provide a sufficiently uniform distribution of heat to achieve good fusion for a range of pipe diameters. The outer ends of the bores would be plugged to hold the heaters in place and the heaters, peripherals and controls would be connected in the central opening in order to hold the diameter of the heater to a minimum. In such a configuration the heater spacing for larger diameter pipes of this range becomes so great that uniform temperature distribution is not possible. Therefore, the heater is good for only a small range of pipe diameters.
It is, therefore, an object of this invention to provide a heater plate which accommodates differential expansion of the heaters and the heater body. Another object of this invention is to provide a heater plate which facilitates uniform temperature distribution over a wide area. It is also an object of this invention to provide a heater plate which can be repaired in the field. A further object of this invention is to provide a heater plate which will accommodate a wide range of pipe diameters. And it is an object of this invention to provide a heater plate which is relatively inexpensive.